Fix QA issues of the devtools README (#5963)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5963

Add new files in the "Directory structure".
And also remove an old statement. Since `-DEXECUTORCH_BUILD_DEVTOOLS=ON` is set in the build_example_runner.sh, and not in a command in the readme, so it no longer makes sense to call it out in this readme.

Reviewed By: Gasoonjia

Differential Revision: D64017662

fbshipit-source-id: 1ee8b086180c7f995ff3306ebceb4afcff934214